The present invention relates to an improved process for preparing perfluoroalkyl aryl sulfides and to two novel perfluoroalkyl aryl sulfides.
Aromatic compounds carrying fluorine-containing substituents are intermediates for the synthesis of agrochemicals, pharmaceuticals, dyes, and polymers. The trifluoromethylthio group in particular is included in the development of novel biologically active compounds and novel dyes, since it has high chemical stability and is the most lipophilic known substituent E.g., R. E. Banks and K. C. Lowe, "Fluorine in Medicine in the 21.sup.st "Century", Chemserve Ltd., Manchester (1994); S. B. Walker, "Fluorine Compounds as Agrochemicals", Fluorochem Ltd., Old Glossop (U.K.) (1990); EP-A 374,061; J. H. Clark, D. Wails, and T. W. Bastock, "Aromatic Fluorination", CRC Press, Boca Raton, Fla. (1996), page 119. 2-Hydroxyphenyl trifluoromethyl sulfide, for example, is required for preparing certain herbicides. E.g., U.S. Pat. No. 4,581,059. Chlorine-, nitro-, and amino-substituted phenyl trifluoromethyl sulfides are employed as intermediates for pesticides. E.g., DE-A 3,737,984 and DE-A 3,737,985.
Hitherto, a number of methods for preparing perfluoroalkyl aryl sulfides have been disclosed. Most of these methods require expensive and/or toxic reagents, which is a disadvantage for industrial production. Pentafluoroethyl and long-chain perfluoroalkyl radicals can be introduced into aromatic thiols by ion-radical and cationic reactions with perfluoroalkyl iodides. E.g., J. General Chem. USSR, 53, 2254 (1983) and J. Org. Chem. USSR, 13, 1985 (1977). However, perfluoroalkyl iodides have the disadvantage that they must be prepared in a complicated manner and are therefore expensive.
A more recent method for preparing C.sub.1 derivatives in which a thermally induced decarboxylation of potassium trifluoroacetate is carried out in the presence of aryl sulfides has been reported. E.g., Tetrahedron Lett., 37, 9057 (1996). Although this process constitutes a considerable advance on the earlier methods, the yields--in most cases only about 50% of theory--are still unsatisfactory, and the process has hitherto been limited to introducing the trifluoromethyl group. Furthermore, this process is virtually impossible to carry out on an industrial scale, since the strongly exothernic reaction is virtually impossible to control. The entire reaction mixture is described as being heated rapidly by bringing it into contact with a hot heat exchange medium. A strongly exothermic reaction then results. No details about the pressure are given. Consequently, one skilled in the art must assume that the operation was carried out under atmospheric pressure.